Lasers Speak to Subs

19 Sep 2006

Communicating with subs underwater is beyond tough. Sound moves through seawater in very strange ways, with water temperature, salinity, and density speeding up and slowing things down -- garbling conversations in the process. Electromagnetic transmissions (like radio) are no better -- the sea has some funky electrical conductivity. During the Cold War, sub authority Joe Buff notes, the Navy managed to get super-simple, one-way messages to its subs, with a pair of giant (28-mile!) extremely low frequency transmitters, based in the Midwest. But those transmitters were shut down, a few years back.The Navy's new idea is to get specially-tuned lasers to handle the job, instead. The service has handed out a pair of small business innovation research contracts to Bothell, WA's Aculight Corporation and Bedford, MA-based Q-Peak to build blue-green, quick-burst lasers for transmitting messages across the deep. Acluight, for example, wants to use a combination of semiconductor and fiber lasers to produce a low power beam (around 10 watts) at about 532nm spectrum range. The idea is to get pulses as quick as half a nanosecond, repeating as much as 10 million times per second.Blue-green lasers have been discussed for a while as potential sub-talkers, with good reason. Seawater has a lot of organic junk floating around inside, which makes it "turbid" -- "nearly opaque to light over much of any distance," Buff explains.

Blue-green light's frequency is best at penetrating through this turbidity, given the mix of sizes in microns of the particles and other stuff that prevents seawater from being transparent. (Of course, some areas such as the Bahamas are famous for the clarity of their water, but this is very much the exception, not the rule, globally speaking.) This same turbidity is essential to giving submarines their invisibility while submerged, so it's a double edged sword.